This is the most helpful article I have found yet on the problems at earthquake-damaged nuclear plants. As one can imagine, it is a lot more sensible than some of the garbage in the general media.

It cleared up one point of confusion I had - I was not sure why there was still heat generation after the control rods slammed down, killing the fission process. But apparently there are a number of intermediate fission products created that continue to decay for several days, producing about 3% of the heat of the full fission process. This heat is what boiled away the water in the reactor vessel once flow of cooling water stopped. It is this boiling that led to the necessity to release steam (to reduce pressure in the reactor vessel). It was this steam that was partially disassociated into hydrogen and oxygen, which led to the explosion.

One fact that has been lost in all the hype, and may continue to be lost, is that the earthquake alone (which was 7 times larger than the plant was designed for) was necessary but not sufficient to lead to the current problems. Everything probably would have been fine had it not been for the tsunami knocking off all the diesel generators the plant used in an emergency to keep the colling pumps running. Apparently the generators they rushed to the site later could not be used due to various incompatibilities, the type of real-world frustrating problem that will be immediately recognizable to any engineer who has a troubleshooting background.

Update: Unfortunately, the author may have been overly optimistic. The author implied the pile would stop producing new heat after a few days, but that does not seem to be the case, particularly since spent fuel rods apparently have to be kept in water to keep them cool months or years after they were in service. With the apparent rupture of the main presure vessel around the core, all bets would seem to be off in terms of containing the most harmful radioactive elements.

I did troubleshooting at a refinery for years, and almost every time the worst disasters were from improbable event and/or screwup after improbable event. The human mind seems to be unable to really grasp just how screwed up things can get. The novel Jurassic Park was as much about this problem as it was about dinosaurs.

Update #2: This is the piece that was missing from the earlier linked report:

The sharp deterioration came after a frantic day and night of rescue efforts focused largely on the No. 2 reactor. There, a malfunctioning valve prevented workers from manually venting the containment vessel to release pressure and allow fresh seawater to be injected into it. That meant that the extraordinary remedy emergency workers had jury-rigged to keep the nuclear fuel from overheating no longer worked.

As a result, the nuclear fuel in that reactor was exposed for many hours, increasing the risk of a breach of the container vessel and more dangerous emissions of radioactive particles.

By Tuesday morning, Tokyo Electric Power said that it had fixed the valve and resumed seawater injections, but that it had detected possible leaks in the containment vessel that prevented water from fully covering the fuel rods.